Factor common code out of CMakeDetermine(ASM|C|CXX|Fortran)Compiler

The compiler candidate list selection and search code for C, C++, ASM,
and Fortran languages was duplicated across four modules.  To look for
compilers adjacent to already-enabled languages the C and CXX modules
each used _CMAKE_USER_(C|CXX)_COMPILER_PATH and the ASM module used
_CMAKE_TOOLCHAIN_LOCATION.  Since commit 4debb7ac (Bias Fortran compiler
search with C/C++ compilers, 2009-09-09) CMake prefers Fortran compilers
matching the vendor and directory of an enabled C or C++ compiler.

Factor out the common functionality among the four languages into a new
CMakeDetermineCompiler module.  Generalize the Fortran implementation so
that all languages may each use the vendor and directory of the other
languages that have already been enabled.  For now do not list any
vendor-specific names for C, C++, or ASM so that only the directory
preference is used for these languages (existing behavior).
Prefer generic system compilers by default for C, C++, and Fortran

Teach CMake to prefer the system default compiler automatically when no
compiler is specified.  By default use "cc" for C, "CC" for C++, and
"f95" for Fortran.  Load a new Platform/<os>-<lang>.cmake module to
allow each platform to specify for each language its system compiler
name(s) and/or exclude certain names.

Create Platform/(CYGWIN|Darwin|Linux|Windows)-CXX.cmake modules to
specify "c++" as the system C++ compiler name for these platforms.  On
systems that use case-insensitive filesystems exclude C++ compiler names
that are distinguished from C compiler names only by case.

This will change the default compiler selection for existing build
scripts that do not specify a compiler when run on machines with
separate system and GNU compilers both installed in the PATH.  We do not
make this change in default behavior lightly.  However:

(1) If a given build really needs specific compilers one should specify
    them explicitly e.g. by setting CC, CXX, and FC in the environment.

(2) The motivating case is to prefer the system Clang on newer OS X
    systems over the older GNU compilers typically also installed.  On
    such systems the names "cc" and "c++" link to Clang.  This is the
    first platform known to CMake on which "c++" is not a GNU compiler.
    The old behavior selected "gcc" for C and "c++" C++ and therefore
    chooses GNU for C and Clang for C++ by default.  The new behavior
    selects GNU or Clang consistently for both languages on older or
    newer OS X systems, respectively.

(3) Other than the motivating OS X case the conditions under which the
    behavior changes do not tend to exist in default OS installations.
    They typically occur only on non-GNU systems with manually-installed
    GNU compilers.

(4) The consequences of the new behavior are not dire.  At worst the
    project fails to compile with the system compiler when it previously
    worked with the non-system GNU compiler.  Such failure is easy to
    work around (see #1).

In short this change creates a more sensible default behavior everywhere
and fixes poor default behavior on a widely-used platform at the cost of
a modest change in behavior in less-common conditions.
Merge topic 'select-compiler' into next

7e58e5b Prefer generic system compilers by default for C, C++, and Fortran
796e337 Factor common code out of CMakeDetermine(ASM|C|CXX|Fortran)Compiler